sedimentary
Chert (Flint Nobule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Brown/Grey with a weathered white cortex, Luster: Waxy or dull when weathered, Crystal Structure: Microcrystalline/Cryptocrystalline (Hexagonal), Cleavage: None (Conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown/Grey with a weathered white cortex, Luster: Waxy or dull when weathered, Crystal Structure: Microcrystalline/Cryptocrystalline (Hexagonal), Cleavage: None (Conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica in sedimentary rock layers, often limestone or chalk. It can be biochemical (accumulated sponge spicules or radiolarians) or chemical. These often date back to the Cretaceous or Jurassic periods.
Uses & applications
Historically used for stone tools (knapping) and as strike-a-lights for fire starting. Modernly used in industrial abrasives and as road aggregate.
Geological facts
Chert and flint are essentially the same mineral; 'flint' is often specifically used to describe chert found in chalk or limestone beds. Because it fractures with extremely sharp edges, it was a primary material for prehistoric arrowheads and scrapers.
Field identification & locations
Identify in the field by its smooth, waxy fracture surface (conchoidal) and its ability to easily scratch glass. Commonly found in chalk downs, riverbeds, and as nodules in limestone outcrops.
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